Production of oil, condensate and gas from reservoirs located subsea, more and more often is from wellheads located on the seabed. A subsea XT (Christmas tree) is located on such wellheads, providing valves for control between the wellhead and the choke valve. The XT valves are typically either open or closed for production flow. The valves are usually gate valves controlled via a reciprocating function of an actuator. The actuator can be designed as a spring loaded piston where the piston rod is extended to open the valve when hydraulic pressure or electric power is applied to the actuator.
The piston rod is typically coaxial to the valve stem, and said elements operate as one element and they can be one single element or be divided into sections. The valve stem connects the actuator to a valve element, often termed a gate, in a pressure housing containing the valve element. At least a part of the valve stem is exposed to the pressure of the pressure housing of the valve as it may be extended into said pressure housing. The valve stem can be a single or several coaxial lengths.
Sealing against the pressure of the valve housing takes place between the valve stem and a bonnet surrounding the valve stem. In a typical sealing arrangement the seals comprise primary and secondary dynamic seals, and also further seals. Currently the secondary seal is held in a carrier. This results in the need for a static seal on the outer diameter surface of the carrier, toward the bonnet. Such prior design has evolved under considerations with respect to assembly, maintenance and replacement. However, the prior design results in several seals of different design, geometry and pressure integrity, resulting in a voluminous and complex seal arrangement providing several paths for leakage and making testing and qualification of each seal very difficult. Therefore, a demand exists for a new valve stem seal arrangement providing improvements with respect to number and length of leakage paths, number of different designs and geometries providing simplified seal arrangement, prolonged service life, allowing smaller dimensions and simplifications with respect to testing, qualification and maintenance. The objective of the present invention is to meet the demand.